The application relates generally to coating of metallic substrates and more specifically to the use of an interlayer to enhance electrodeposited aluminum coating adhesion on aluminum alloys.
Aluminum alloys in general, and high strength aluminum alloys in particular, are prone to localized corrosion. The presence of precipitates such as intermetallic particles is responsible for pitting corrosion susceptibility of these alloys. Additionally, secondary phases depositing in grain boundaries promote inter-granular corrosion, leading to exfoliation failure. The aluminum matrix of these alloys is chemically reactive and naturally forms an oxide film in the presence of water and air. The oxide is partially protective to the substrate, but offers little resistance to pitting corrosion that arises from the electrochemical potential difference between the matrix and the intermetallic phases. It is known that pure aluminum is significantly resistant to corrosion, in particular, localized corrosion such as pitting. Thus, coating aluminum alloy components with pure aluminum is an effective method to protect the aluminum alloy structures.
Electrodeposition of aluminum from aqueous solutions is not possible because the electronegativity of aluminum in relation to water is such that hydrogen will form in deference to aluminum deposition in a plating bath. The only commercialized aluminum electroplating technology in the U.S. is Alumiplate™, which employs a bath that is pyrophoric (triethylaluminum in solvent toluene) and operates above room temperature (at 100° C.). Such aluminum electroplating can be difficult and dangerous to implement due in part to the pyrophoric nature of the plating chemistry and use of organic solvents such as toluene. Toluene is currently listed by the U.S. Environmental Protection Agency (EPA) as a hazardous air pollutant (HAP).
Other advanced coatings processes have been developed but each has shortcomings. Thin film chemical vapor deposition (CVD), physical vapor deposition (PVD), and ion vapor deposition (IVD) cannot produce dense coatings. Dense coating is preferred as a corrosion protection barrier of the substrate. Recent advances in ionic liquids and related processes have shown promise for depositing dense aluminum coatings. Electroplating aluminum in room temperature ionic liquids has advantages of non-line-of-sight, green chemistry and being non-flammable compared with alternative technologies such as the Alumiplate process and IVD.
It is challenging to attain an adherent metallic coating on aluminum alloys via electroplating due to extremely rapid formation or re-formation of aluminum oxide. Specifically, aluminum alloys are chemically reactive with water and air, forming a native alumina film in ambient conditions. It is believed that removal of the oxide film is necessary for depositing adherent Al coatings. Due to the fast formation of aluminum oxide, it is common to deposit a thin zinc coating in an alkaline zincate solution prior to electroplating. The zinc immersion coating is deposited onto the aluminum alloys via the exchange reaction between Al and zincate ions. Powdery zinc deposit and inadequate surface coverage is common for such an immersion coating due to the nature of the reaction and the surface heterogeneity of Al alloys, therefore, double immersions with acid (HCl) etching in between immersions are a standard practice prior to electroplating. Partial dissolution of the zinc immersion coating occurs spontaneously in acidic plating baths and it allows a metallic coating to be partially deposited on aluminum substrates. The lack of acid in the ionic liquid plating bath makes the spontaneous dissolution of the zinc coating impossible, which can lead to adhesion and potentially corrosion issues. It is therefore desirable to remove the zinc coating in an ionic liquid solution by electrolytic etching prior to electroplating. It is also desirable to use an optimized interlayer composition and morphology to maximize deposition of aluminum onto the substrates with superior coating/substrate adhesion.